Biostratigraphy of the South Caspian Basin: Insights from Offshore and Onshore Wells and Outcrops in Turkmenistan and Iran

A biostratigraphic comparison was conducted between the offshore and onshore regions of the Gorgan-Dag-Ekerem uplift zone in Turkmenistan and offshore Iran, utilizing published data from the Gorgan Plain in northern Iran. The study focuses on the Pliocene Red-bed, Akchagylian, and Quaternary deposits in the South Caspian Basin. Pliocene deposits hold significant economic and scientific interest due to their commercial reservoir potential beneath thick Quaternary formations. The practical study of Quaternary sediments is crucial for selecting suitable engineering and construction sites for offshore platform installation and identifying safe locations for exploratory wells. Industrial hydrocarbons have also been extracted from the Quaternary deposits (Apsheron stage) in Turkmenistan. A total of 533 core and drill-cutting samples were analyzed from shallow and deep exploration wells across three onshore and seven offshore structures in Turkmenistan and Iran. This included 161 samples from shallow offshore wells, 209 from deep offshore wells, and 163 from onshore near-coastal structures. Abundant ostracod assemblages were identified, enabling detailed biostratigraphic characterization of the Pliocene Red-bed strata, Akchagylian deposits, and Quaternary formations. Pliocene deposits exhibited relatively low microfaunal diversity. The Quaternary deposits displayed a rich assemblage of predominantly brackish ostracods, along with euryhaline and freshwater species.